1. Field of the Invention
The present invention relates to novel organic dye compounds, and more particularly, to trimethine cyanine dyes which are useful in high-density optical recording media.
2. Description of the Prior Art
As coming into this multi-media age, the following optical recording media have been greatly focused:
(i) compact disc recordable or CD-R, a write-once memory using compact disc, and
(ii) digital versatile disc recordable or DVD-R, a write-once memory using digital video disc.
Optical recording media can be roughly classified into inorganic optical recording media which have recording layers composed of inorganic substances such as tellurium, selenium, rhodium, carbon, or carbon sulfide; and organic optical recording media which have recording layers mainly composed of light absorbents containing organic dye compounds.
Among such optical recording media, organic optical recording media can be usually prepared by dissolving a cyanine dye in an organic solvent such as 2,2,3,3-tetrafluoro-1-propanol (abbreviated as xe2x80x9cTFPxe2x80x9d hereinafter), coating the solution on the surface of a polycarbonate substrate, drying the coated solution to form a recording layer, and sequentially forming and attaching on the surface of the recording layer (i) a reflection layer made of a metal such as gold, silver or copper, and (ii) a protection layer made of an ultraviolet ray hardening resin. When compared with inorganic optical recording media, organic ones may have the drawback that their recording layers may be easily changed by environmental lights such as reading- and natural-lights. Optical recording media, however, have the merit that they can be made into optical recording media at a lesser cost because their recording layers can be directly formed by coating light absorbents in a solution form on the surface of substrates. Further, organic optical recording media, which are composed of organic materials, are now mainly used as low-cost optical recording media because they are substantially free of corrosion even when contacted with moisture or sea water and because information, stored in optical recording media by a fixed format, can be read out by using commercialized readers by the establishment of thermal-deformation-type optical recording media as organic optical recording media.
What is urgently required in organic optical recording media is to increase their storage capacity to suit for this multi-media age. The research for such an increment, which is now being eagerly continued in this field, is to shorten the wavelength of a laser beam for writing information from 775-795 nm, that is irradiated by conventional GaAlAs semiconductor lasers, to a wavelength of 700 nm or shorter. However, since most of conventional cyanine dyes explored for CD-Rs could not appropriately write and read information by using a laser beam with a wavelength shorter than 700 nm when used in high-density optical recording media such as DVD-Rs, the cyanine dyes now used could not fulfil the need for high-storage density required in many fields.
As another causative for spoiling the high-storage density of organic optical recording media, there may exist problems of the thermal decomposition and the heat resistance of dyes. In organic optical recording media, pits are formed by using heat generated when dyes absorb a laser beam and then melt and decompose. However, most of conventional cyanine dyes have a rather lower decomposition point, and this results in the problem that the part around the pits and other pit-less part on the recording surface may be easily deformed by the accumulated heat which is generated when the dyes are exposed to a reading laser-beam for a relatively-long period of time because the cyanine dyes have a relatively-low heat resistance.
High quality products must be provided in large quantity and low price to make high density optical recording media such as a DVD-R fix, as an information recording means for multi-media age, in place of papers. For the purpose, it is necessary to efficiently coat light absorbents on a substrate and make optical recording media with a good recording characteristic and higher stability. Light absorbents which easily dissolve in organic solvents are essential, and more particularly, the development of light absorbents, which less pollute environment and easily dissolve in non-halogen solvents, has been desired. Although various light absorbents were provided and some of them have been actually used, no light absorbent, which can satisfy both the desired light characteristic and solubility, has been realized.
In view of the foregoing, the object of the present invention is to provide organic dye compounds which exert satisfactory solubility and heat resistance when used in high-density optical recording media.
To attain the above object, the present inventors eagerly studied and screened compounds. As a result, they found that specific trimethine cyanine dyes (may be called xe2x80x9ccyanine dyesxe2x80x9d hereinafter) which substantially absorb a visible light with a wavelength of shorter than 700 nm, are obtainable through a step of reacting benzoindolenium compounds bearing a reactive methyl group with benzoindolenium compounds bearing a suitable leaving group. They also found that, when compared with conventional related compounds, most of the cyanine dyes of the present invention have the following characteristics: They have significantly-high solubility in organic solvents which are frequently used in preparing optical recording media, and more particularly, non-halogenated solvents, have decomposition points over 272xc2x0 C., and have relatively-high heat resistances. The present inventors confirmed that the trimethine cyanine dyes form minute pits stably on the recording surfaces and at a relatively-high density when irradiated with a laser beam at a wavelength of shorter than 700 nm in optical recording media. The present invention was made based on the creation of novel organic dye compounds and the discovery of their industrially useful characteristics.